There are several ADs out there and they suit people in different ways. I have been on them for the best part of 7 years on and off. I am on Escitalopram now and was on citalopram. Mostly for anxiety for me and they work well, although am due a meds review soon and im worried they are going to suggest i stop.
Like others, the first few weeks on citalopram were grim but not unmanagable and then it settled down. The second time i went on them i was prepared for this and asked for diazepam to counter it, but the first time i didn't need it. I skipped from euphoric to panic attack in seconds. Felt a bit nauseous like you feel when you first start taking contraceptive pill.
When i was on 40mg citalopram i was most definately zoned out, i could have walked in front of a bus and not been bothered, not uncaring but just not aware of the danger. I was on this for a while and DP said it was very much a case of the lights on but no one at home. This is why i changed to escitalopram as it allowed me to have a lower dose. Saying that 40mg is a HIGH dose and they definately wont start you on that. 20mg is considered the clinical optimum for most people. They often start you on 10mg then move up to try and avoid the side effects in the first week.
I DID put on weight but i also had a mirena coil fitted at the same sort of time and i put it down to that more than the ADs.
What you are prescribed will depend on what sort of issues you are having, so some meds suit depression more than anxiety and vice verse although you will almost certainly be presribed and SSRI which works on the seretonin producing cells in your brain. Helping them to utilise seretonin more effectively and produce more of their own, rather than just giving you seretonin which wont cross the blood brain barrier (which is why St John's wort is a load of shit and doesn't work! butthats another thread).
Exercise really does help, i have started running and it makes such a difference, but i totally understand the lack of energy at the moment, hopefully when you start to feel more alive you will find the energy for some sort of exercise, but forcing yourself out of the house for a walk for half an hour is something and will definately help. I strongly believe that the weather makes a big difference too - i often get told off for saying this because depression is a serious illness and it sounds minimalising but i promise you i feel so much better in the summer. I think the winter just makes people feel even worse.
Let us know what the doc says, we are here to hold your hand and help you through this.